This invention relates generally to mediums for recording thereon information signals such as phonograph and video disc mediums and to materials therefor, the signals thus recorded being reproducible from the mediums. More particularly, the invention relates to a phonograph or video disc which is fabricated from a copolymer containing an unsaturated nitrile component and has highly desirable properties such as antistatic property, abrasion resistance, impact resistance, heat stability, transparency, and homogeneity.
The material mostly used for phonograph discs at present is a vinyl chloride -- vinyl acetate copolymer of low molecular weight. Furthermore, as materials for video discs being developed recently, vinyl chloride resins produced by emulsion polymerization are being considered. Phonograph and video discs in which vinyl chloride resins are used are accompanied by a number of problems as described below.
1. The first problem is that a phonograph disc in which a vinyl chloride -- vinyl acetate copolymer is used or a video disc in which a vinyl chloride (VC) resin produced by emulsion polymerization (hereinafter referred to collectively as "VC resin record discs") exhibits a conspicuous electrostatic charge accumulating characteristic (hereinafter referred to as "electrostatic characteristic"). This electrostatic characteristic is still of a remarkable degree even in the case where an antistatic agent has been applied as a coating on the surface of the phonograph disc or has been kneaded into the disc resin.
As is known, an electrostatic characteristic of this degree gives rise to attraction of dust in the air and the surroundings to the phonograph disc surface and thus into the recording groove. As a consequence of this dust in the groove, not only is noise generated at the time of reproducing (playing) of the record disc, but the serviceable life of the reproducing stylus is shortened, and the groove walls are damaged, whereby the serviceable life of the disc itself is reduced. While various cleaners for removing this dust in the disc groove are being sold on the market, they are troublesome because they must be used every time a disc is to be played. Furthermore, in the case of a video disc, the presence of dust in the disc groove has a greater deleterious effect because of the high-density recording.
2. The second problem is that vinyl chloride resins, particularly vinyl chloride resins produced by emulsion polymerization, have poor heat stability and their decomposition temperatures are close to their processing temperatures, whereby these resins readily decompose or undergo degeneration. In order to improve the heat stability of these resins, it is a common practice to add stabilizers thereto, but this measure cannot be said to be completely satisfactory. Moreover, a stabilizer agent used in this manner bleeds out of the surface of the record disc after forming and becomes a cause of noise.
3. The third problem is that, in order to produce resins of good formability and workability, these VC resins are produced with a low molecular weight and as copolymers with vinyl acetate. For this reason, these resins have poor heat resistance and poor abrasion resistance and, in the form of a record disc, are easily warped and easily scratched or scored.
Heretofore, new high-polymer materials in which the above described problems accompanying vinyl chloride resins have been overcome have been proposed. However because they entail other problems, they have not replaced VC resins for recording discs. For example, as is known, polystyrene resins have poor abrasion resistance and poor impact resistance, while polymethacrylate resins have poor fluidity at forming or molding, poor abrasion resistance, and poor impact resistance.